Magnetic field sensing has become a well-established method for tracking the coordinates of objects involved in medical procedures. Position sensors affixed to the objects being tracked are typically used to measure the relative strengths of externally-generated magnetic fields. These magnetic field measurements are then used to derive object coordinates. Systems operating in this manner are disclosed, for example, in U.S. Pat. Nos. 5,391,199, 5,443,489, and 6,788,967 to Ben-Haim, in U.S. Pat. No. 6,690,963 to Ben-Haim, et al., in U.S. Pat. No. 5,558,091 to Acker et al., and in U.S. Pat. No. 6,177,792 to Govari, all of whose disclosures are incorporated herein by reference.
In alternative systems for magnetic tracking, objects being tracked radiate a magnetic field which is sensed by external receivers. For example, U.S. Pat. No. 5,099,845, to Besz et al., whose disclosure is incorporated herein by reference, describes a location determining device, which has a radiating element forming part of the device to be inserted into an object (such as a human body). The element radiates a signal, which is detected by at least one receiving element. The received signal energy level is used to measure the distance of the radiating element from the receiving element, which is then indicated to an operator in order to locate the device within the object.
U.S. Pat. No. 5,762,064, to Polvani, whose disclosure is incorporated herein by reference, describes a medical magnetic positioning system and method for determining the position of a magnetic probe inside the body. At least two spaced magnetometers are fastened to an area on an external portion of the body proximate to the desired location of the probe inside of the body. The three-dimensional magnetic field of the probe is detected at the magnetometers, and the location of the probe is determined in accordance with the location of the detected three-dimensional field.
U.S. Pat. No. 6,453,190 to Acker et al., whose disclosure is incorporated herein by reference, describes a system in which a field transmitter capable of either detecting or generating a magnetic field is fixed to an instrument at an arbitrary position with respect to a feature of the instrument. A relationship between the instrument feature and the field is calibrated so that the position of the instrument feature may be determined based on the field.
Accurate magnetic field position sensing may be hampered when magnetically-responsive objects, such as metallic tools, enter the space of the magnetic fields. Eddy currents induced in the metallic objects generate parasitic fields that may cause errors in position measurements. Methods for correcting or avoiding these errors have been suggested by prior art. For example, U.S. Pat. Nos. 4,849,692 and 4,945,305 to Blood, whose disclosures are incorporated herein by reference, describe a tracking system that overcomes problems of eddy currents by using pulsed DC magnetic fields. Sensors which are able to detect DC fields are used in the system, and eddy currents are detected by utilizing their decay characteristics and amplitudes.
U.S. Pat. No. 6,201,987, to Dumoulin, whose disclosure is incorporated herein by reference, provides systems for compensating for eddy currents using alternating magnetic field generators. In a first system, compensation for eddy currents is provided by first calibrating the system free from eddy currents, and then modifying the fields generated when the eddy currents are detected. In a second system the eddy currents are nullified by using one or more shielding coils placed near the generators.
U.S. Pat. No. 5,767,669 to Hansen et al., whose disclosure is incorporated herein by reference, describes a method for detecting eddy current distortions in position measurements. The method uses pulsed magnetic fields for position sensing. The rate of change of a sensed field is measured in order to detect eddy currents. Compensation for the eddy current distortions is provided by adjusting the duration of the magnetic pulses.
U.S. Pat. No. 6,373,240 to Govari, whose disclosure is incorporated herein by reference, provides a method for detecting a parasitic field. Parasitic fields generated by a driving signal are shifted in phase relative thereto. A computer process generates driving signals over a range of frequencies. At each frequency, a phase shift is measured in a combined position signal comprising the driving signal and the parasitic field. The computer process determines a frequency that produces a minimum phase-shift, and thus a minimum effect of the parasitic fields. This frequency is used to calculate the position of the object. Alternatively, measurements of the combined signal are made at a plurality of frequencies. The values obtained are used to solve a plurality of simultaneous equations comprising the position signal as one of the unknowns in the equations.
U.S. Pat. No. 6,172,499 to Ashe, whose disclosure is incorporated herein by reference, provides a method for measuring the position and orientation of a receiving antenna. Two or more transmitting antennae of known location and orientation relative to one another are driven by AC excitation. The receiving antennae measure the transmitted AC magnetic fields plus distortions caused by metal objects. Signal processing means are used to ascertain relative values of phase-separated components of the fields to substantially eliminate position errors caused by eddy current distortion.
U.S. Patent Publications 2004/0254453 and 2004/0239314 to Govari, whose disclosures are incorporated herein by reference, provide a position sensing method comprising detection of harmonic frequencies of parasitic fields. A parasitic field generated by a magnetically-responsive element is detected by a pattern of harmonic frequencies indicative of the element. The detected pattern of frequencies is removed from a position signal received by the probe and the resulting clean signal is used to calculate the probe position.
U.S. Patent Publication 2004/0102696 to Govari, whose disclosure is incorporated herein by reference, provides a further method for compensating for parasitic fields. Reference elements placed at known positions near a probe receive position signals indicative of measured reference positions. The measured reference positions differ from the known reference positions due to the interference of parasitic fields. The differences between the measured and the known positions provide a correction factor, which is used to correct a measured position of the probe.
Magnetic-based position sensing systems currently available include products such as the CARTO™ EP Navigation and Ablation System and the LASSO™ Circular Mapping Catheter, produced by Biosense-Webster (Diamond Bar, Calif.).